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Old Jul 18, 2009, 01:41 PM
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United States, AL
Joined Apr 2007
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Quote:
Originally Posted by George Shering
.... There are two types of drag, profile drag and induced drag. .....
I know it's off topic, but there are three types of drag - you forgot parasite drag.
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Old Jul 18, 2009, 02:22 PM
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Chipping Norton, UK
Joined Jan 2005
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Flight Data

Quote:
Can you guys post some fly data on the cells you have ?

I will appreciate if someone post here F5B discharge graph of the Neu 35/70 C.
I will run 1512 1D 4S setup and want to buy the best batteries.
Only logged data requested.
Data from a practise flight. The set up was a Enigma, 1512/1D 6.7:1, 17x18 green +7 hub, Overlander Extreme (Fullymax) 4s 4400. Not sure of the air temp but I'd guess at about 25 deg C.
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Old Jul 18, 2009, 02:35 PM
The Kid
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Detroit, MI
Joined Dec 2007
2,583 Posts
Quote:
Originally Posted by biskit
I know it's off topic, but there are three types of drag - you forgot parasite drag.
Though i could be mistaken, induced drag and profile drag are two kinds of parasitic drag...
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Old Jul 18, 2009, 02:49 PM
The props of tomorrow
Varna , Bulgaria
Joined Feb 2007
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Thanks a lot Steve !!! What is the price of one 4S pack ? Also what is the C rate ?
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Old Jul 18, 2009, 03:22 PM
Needs to do 52 legs !!
jjmouris's Avatar
Verenigd Koninkrijk, Fareham
Joined Aug 2008
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Georgi, try www.overlander.co.uk

But i recommend www.thunderpowerrc.com

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Old Jul 18, 2009, 04:45 PM
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Scotland
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Quote:
Originally Posted by biskit
I know it's off topic, but there are three types of drag - you forgot parasite drag.
Yes, you are right. I just rolled profile drag and parasitic drag into the same thing. Profile drag is normally applied to wings and parasitic drag to fuselages and bits hanging out such as tail servos ( I am just changing a two servo in the tail B6 into a single fuselage servo partly for drag reasons). But both these drags are just a consequence of pushing the thing through the air and are independent of weight. The induced drag is a consequence of creating lift and goes up with the weight, quite sharply as the coefficient of lift approaches one in turns. Drag times speed is watts so wastes our watt.minutes. So the lighter the better as Eric said.
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Old Jul 18, 2009, 05:18 PM
Needs to do 52 legs !!
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George, the lighter the better except for two things;

#1 The angle you have to dive to keep the speed will depend on the weight of the plane and going very high only to dive down will mean more track miles and a higher number of energy conversions.

#2 Displacing air with something relatively dense seems more easy then with something light. Example.... feather vs lead falling through the air although it could be argued the feather has more drag.

Certainly real gliders carry water ballast on good thermal days to increase travelling speed. If a lighter wingloading was always better, this would not happen.

Sorry for the OT

Joe
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Old Jul 18, 2009, 07:36 PM
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Quote:
#1 The angle you have to dive to keep the speed will depend on the weight of the plane and going very high only to dive down will mean more track miles and a higher number of energy conversions.
Except you forgot the height we start from is dependendent on the weight.
This is an energy management event where the initial PE=0 and KE=0 not some glider event where you start from 2000ft and can lug as much mass as your care to take to the start height.
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Old Jul 20, 2009, 12:29 PM
Needs to do 52 legs !!
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David, if you are familiar with real gliding then you should know the start and finish height of a glider in a triangular distance flight (competition) is insignificant. The real travaling speed of the glider is a ratio between gliding in an almost strait line and thermalling in a thermal to gain altitude. If you are heavy, it will take longer to climb to the same altitude in the thermal. This you then have to offset by a better glide speed on the glide. It's all very complicated and basically taking water only works out positively if the lift conditions are good and/or if there is lots of wind.

I can recommend for any F5B pilot to get a book on gliding, inparticular one that involves how to read the air and competition flying. It's very interresting!
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Old Jul 20, 2009, 02:54 PM
Boom goes the dynamite!
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Palo Alto, CA
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Joe any specific book you recommend?
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Old Jul 20, 2009, 05:47 PM
Needs to do 52 legs !!
jjmouris's Avatar
Verenigd Koninkrijk, Fareham
Joined Aug 2008
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It would be in Dutch or German.

In any case, given the decrease in IR with the new generation Lipo's. It would seem we can make do with cheaper lower spec cells rather then the top of the line expensive stuff.

Alternatively we can use smaller capacity cells of the good stuff. The later obviously resulting in a lower weight and thus lower wingloading.

Now, given the same initial performance from both packs (startup power), wich one will give the more constant power for the rest of the climb?!

It will be VERY interresting to see how all this effects total flight performance in F5B.

EDIT try "For a comprehensive description of speed to fly theory read Cross Country Soaring by Helmut Reichmann published by Thompson, a book on competitive sailplane flying essential for all competition pilots."
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Old Jul 20, 2009, 06:25 PM
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Sydney Australia
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Joe

Not to get buried in an old argument, but Dave H. knows a lot about model gliding and probably doesn't need to read a book on the topic.

The point I think Dave was making is that lighter weight in F5B helps with acceleration and getting to course entry height quickly and with minimum energy use as well as with faster turns. Heavier planes will hold their energy through the course better. Its a trade off with a seemingly emerging consensus that ligher is better. Whether that will still be true in adverse weather is a matter for more than weathers (sheep) to consider.
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Old Jul 20, 2009, 06:32 PM
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For lift to drag discussion an excellent explanation is found here:
http://www.jdburch.com/polar.htm
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Old Jul 20, 2009, 07:23 PM
Needs to do 52 legs !!
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David, i know David has been around for a while. There is no disrespect ment towards him or anyone else.

I am simply trying to highlight that REAL gliding is a completely different discipline then model gliding and there is a lot we can learn.

In real gliding there has already been a lot of work done with respect to energy management and how to optimise things. A lot of those things are already present in our model planes but there is still a lot more.

I give you a few examples on how i see things;

We use snap flaps to increase the lift in the turns and have the flaps set to one single setting for the rest of the distance task. Real gliders use full span flaps in half a dozen positions throughout the strait line gliding depending on the flying speed.

The best flying speed while gliding in a real glider is dictated by the conditions. If you are in sinking air, you want to get through this fairly quickly and thus speed up. Visa versa, in good air you want to fly slowly as to minimise height loss and spend more time in the good, possibly even rising air. There are a lot more factors that come into play and real gliders these days have a glide computer including GPS input next to the basic air data inputs that will tell the pilot how fast to fly for the conditions (lift/sink/wind) on a continues and ever changing basis. There is even a final glide calculation that will tell the pilot weather he can get home or not from his current position taking into account expected sink and or lift extrapolated from the previous flying. In the old days a simple version of the sink vs speed principle was put into a simple device called the MacCready ring.

Check this out;
http://www.rigg.btinternet.co.uk/documents/S2F2.htm

In Kiev with the rain and bad weather it seemed high speed was not working (washing off to quickly) and so a much higher and slower profile was adopted by those who where not set in their ways. Despite the low clouds. However we all know that going really fast and relatively low does work in good weather conditions. Just look at how well the Italians are doing with their fast small props!

In terms of weight and wingloading. I flew with +1850 grams in Kiev and managed to do over 40 legs with cuts (as always) having only flown the B1T 4 or 5 times. This was in comparison to the great pilots getting 43-44 if they where lucky. I guess that in the rain the extra weight was not a bad thing. I also countered the weight by having a really big prop to produce lots of lift and less speed.

Certainly i have found to have a little extra weight in the plane on windy days makes for much better wind penetration and thus an overall better performance. Close to this is the chosen descent angle. If you look at the computer data you will see some people have faster speeds / shorter times going one way (A to B vs B to A) due to the wind despite that arguably the optimum is closer to a constant slowly increasing time from leg to leg.

In future i think we will end up with an automated system on the airplanes that will decide where the flaps should be at any given point in time and move them there. This is something i was working on for real gliders but found the prospect of getting it certified prohibative (even for your own personal third hand glider). For us in F5B we start really fast and progressively slow down over the legs, so in theory we should have less flaps (maybe negative) at the start and more and more flaps as time goes on untill the cycle starts again at the next climb. Certainly one single flap setting can not be the optimum for both the fast speed on leg 1 and also for the slow speed on leg 4,5 or 6.

Well, i guess this really is diverging from the subject now.
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Old Jul 20, 2009, 08:01 PM
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Joe,
F5B differs significantly from crosscountry gliding.
In F5B the drag of the model in a striaght line is not really influenced by the wing loading(ie Cd is much the same at the low Cl, both the Cd profile and Cdi are the same)) so the energy for the straight leg is just drag x distance.
So yes a heavier model will lose less speed or use less height. But for the same energy input(watt.minutes) the heavier model will be moving slower or have less height proportional to the extra mass. So net energy gain from the heavier model is zero. But from a time perspective(and F5B is a race) the lighter model will reach the other end quicker because it will travelling faster when it enters the course.
In turn the again the drag is the same for the same Cl irelevant of mass. the distance travelled in the turn is directly proportional to the mass aand the energy loss is also proportional to the mass.
So if we take the horizontal turn, the exit speed is the same for heavy V light model, but the heavy model takes longer because of the extra distance.
So looking at the fundumental physics rather than applying the physics of another discipline leads to a different conclusion.
Cross country gliders whether model or fullsize are interested in L/D. F5B is really mainly about drag( actually it is energy management)
A lighter model in windy and adverse conditions will require a greater change in the flight path than a heavier model to achieve the best result.
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